Magnetic Prussian blue (PB) nanocomposites were synthesized by binding PB to a core of magnetite (Fe3O4) nanoparticles for highly efficient and rapid separation of cesium (Cs+) from aqueous solution. The average particle size of the magnetic PB nanocomposites was 13.6 nm, and they had a high surface area (322.19 m(2)/g), leading to efficient Cs+ adsorption capability. The nanocomposites showed a maximum sorption capacity of 280.82 mg/g at an initial Cs+ concentration of 50 mM, pH 7, and 10 degrees C, which is much higher than those of previously reported PB-based adsorbents for removing Cs+. The adsorption behavior followed pseudo-second-order kinetics and obeyed the Tempkin isotherm. The adsorption capacity of Cs+ on magnetic PB nanocomposites remained consistent even at high ionic competition in the simulated seawater. The obtained magnetic PB nanocomposite is a cost-effective adsorbent and can be easily retrieved from an aqueous solution by a magnet after decontamination of cesium. These results showed that the magnetic PB nanocomposite has extensive applicability for the removal of cesium from aqueous solution.